1. Field of the Invention
This invention pertains to the field of fiber sensors coated with a pressure release material.
2. Description of Related Art
Since the early days of fiber optic sensor development effort, there has been large scale interest in utilizing coated optical fibers to detect signals of interest. A great deal of effort has been expanded in utilizing existing coatings and/or developing new coatings which are sensitive to a specific measurand, such as pressure, electric, magnetic and temperature fields. The general premise has been that an optical fiber coated with a material which is responsive to a specific measurand would be made part of a highly sensitive fiber interferometer, thus allowing for a high sensitivity fiber optic sensor. The biggest hindrance to this effort has been the elusive nature of appropriate coatings with sufficient responsivity to provide highly sensitive fiber optic sensors, such as surveillance grade acoustic arrays. The stability and reliability of coatings in harsh environments has also been of concern.
In order to circumvent this problem, researchers have developed fiber optic sensors which utilize mechanical transducing geometries with significant amounts of the fiber exceeding 100 m of optical fiber attached to the surface of the transducer. The signal of interest is converted to a strain signal by the transducer and is subsequently transferred to the optical fiber bonded to its surface. This approach has especially been very successful in developing high performance fiber optic dynamic pressure or acoustic signals sensors. In fact, it is quite clear that surveillance grade fiber optic acoustic sensors have reached a level of maturity where they have entered production mode and are considered the sensors of choice for various future systems particularly as hull arrays for the next generation of attack submarines.
While the approach noted above has been very successful, there is still significant interest in moving away from the mechanical transducer approach and investigating the possibility of using fiber optic sensors with coated fibers.
Apart from the potential for providing more simple and potentially cheaper systems, other main reasons which make fiber optic sensors employing mechanical transducers unattractive, at it least for certain applications, are: (i) a typical mechanical transducer tends to be relatively large in size, of approximately 0.5 xe2x80x3diameter by 6xe2x80x3 in length, which sets the limit on the array size and geometry, especially true for towed-arrays, (ii) in order to obtain sufficient sensitivity, large amounts of optical fiber exceeding 100 m in length is required on the transducing element, and (iii) first two reasons tend to drive up the overall system cost and also make the overall system more complex. For these reasons, it has been important to continue to explore the utility of fiber optic sensors employing appropriately coated optical fibers. However, as pointed out earlier, coatings with sufficient responsivity to a given measurand field are difficult to find. Table 1, below, summarizes the measured performance of prior art coated optical fiber sensors for detecting pressure signals. It is quite clear that performance of a typical coated fiber sensor does not compare well with a mechanical transducer based fiber sensor.
The normalized responsivity in dB re radians/m-xcexcPa of a sensor shown in col.3 of Table 1, above, is multiplied by the amount of optical fiber to be used in a sensor to provide the overall scale-factor of a given fiber optic sensor. Since the sensor is typically configured in an interferometer geometry, the scale factor units are in radians/xcexcPa, where radians is the fundamental unit of an interferometer and Pascals (Pa) is the fundamental unit of pressure. For instance, a poly-carbonate mandrel-based mechanical transducer utilizing 100 m of fiber has a scale-factor of xe2x88x92125 dB re radians/xcexcPa. If the response of such an interferometric hydrophone is demodulated with an electrooptic system whose noise is about xe2x88x92100 dB re radians/Hz at 1 kHz, then the minimum detectable acoustic signal with that hydrophone would be about 25 dB re xcexcPa/Hz, making it an extremely high performance hydrophone, the sea-state 0 being about 45 dB re xcexcPa/Hz at 1 kHz. On the other hand, the best a coated fiber hydrophone would do is have a minimum detectable signal of about 55 dB re xcexcPa/Hz at 1 kHz. Even this is unrealistic since producing coatings with uniform material characteristics over 100 m of optical fiber is non-trivial and additionally, there is the practical issue of packaging 100 m of coated fiber in a reasonable geometry.
The lack of pressure responsivity of a coated fiber sensor has a lot to do with the coating material and how well it makes contact with the optical fiber. Table 1, above, clearly shows that most materials tried so far do not have sufficient pressure sensitivity and would require an unreasonable amount of fiber length to provide sufficient sensitivity for appropriate acoustic detection.
This invention pertains to a coated fiber pressure sensor wherein the fiber coating is a cellular pressure release material.
Another object of this invention is a fiber sensor coated with a cellular pressure release material which is capable of reducing acoustic radiation of a target.
Another object of this invention is a fiber sensor having an outer layer of a cellular pressure release material in which the pressure field can go to zero at the surface of the material and thus convert the impinging pressure to a displacement in the material.
Another object of this invention is a fiber coated with a pressure release material containing microballoons.
Another object of this invention is a fiber coated with a polymeric material containing microballoons, which sensor is also cheap, disposable and can be more than 50 times more sensitive than prior art coated fiber sensors.
These and other objects of this invention are realized by a fiber coated with a thin layer of a pressure release material containing microballoons having sensitivity to pressure that is more than 50 times more sensitive than prior art coated fiber sensors made part of an optical interferometer or containing an in- fiber grating.